The subject matter disclosed herein relates to detection systems for use in imaging systems, such as X-ray based and nuclear medicine imaging systems.
Diagnostic imaging technologies allow images of the internal structures of a patient to be obtained and may provide information about the function and integrity of the patient's internal structures. Diagnostic imaging systems may operate based on various physical principles, including the emission or transmission of radiation from the patient tissues. For example, X-ray based imaging systems may direct X-rays at a patient from some emission source toward a detector system disposed opposite the source across an imaged volume. Attenuation of the X-rays as they pass through the volume, and through any materials or tissues placed between the source and detector, may be determined and used to non-invasively form images of the interior regions of an imaged patient or object. Such attenuation information may be obtained at various angular displacements to generate depth information coincident with the attenuation information.
In addition, single photon emission computed tomography (SPECT) and positron emission tomography (PET) may utilize a radiopharmaceutical that is administered to a patient and whose breakdown results in the emission of gamma rays from locations within the patient's body. The radiopharmaceutical is typically selected so as to be preferentially or differentially distributed in the body based on the physiological or biochemical processes in the body. For example, a radiopharmaceutical may be selected that is preferentially processed or taken up by tumor tissue. In such an example, the radiopharmaceutical will typically be disposed in greater concentrations around tumor tissue within the patient.
In the context of PET imaging, the radiopharmaceutical typically breaks down or decays within the patient, releasing a positron which annihilates when encountering an electron and produces a pair of gamma rays moving in opposite directions in the process. In SPECT imaging, a single gamma ray is generated when the radiopharmaceutical breaks down or decays within the patient. These gamma rays interact with detection mechanisms within the respective PET or SPECT scanner, which allow the decay events to be localized, thereby providing a view of where the radiopharmaceutical is distributed throughout the patient. In this manner, a caregiver can visualize where in the patient the radiopharmaceutical is disproportionately distributed and may thereby identify where physiological structures and/or biochemical processes of diagnostic significance are located within the patient.
In the above examples of imaging technologies, a detector is employed which converts incident radiation to useful electrical signals that can be used in image formation. Certain such detector technologies employ a silicon photomultiplier (SiPM), which is a single anode device containing a number of microcells, and which are useful for detecting optical signals generated in a scintillator in response to incident radiation. One issue that may arise is, in certain detector technologies where SiPMs are employed, the gain of the respective detection elements may be temperature dependent. Such temperature related variation in gain may be problematic in imaging applications. In addition, the SiPM may show temperature sensitivity due to doping variations, or the SiPM may age due to radiation exposure or other affects that can cause gain drift. Even though temperature is known to be the main cause of SiPM gain drift, those other factors can cause uncertainty or error in knowing the gain of the SiPM, therefore preventing the ability to accurately measure gamma ray energy.
To address these affects, certain conventional approaches to monitor the temperature include using sensors, such as thermistors, to compensate the bias voltage (Vbias) to maintain constant over-voltage and account for overall gain drift. However, these conventional approaches typically employ the temperature sensors separate from the SiPM, and are placed in proximity to the SiPM. As a result, the temperature sensors do not measure the actual temperature of the SiPM and so drift in gain is typically not accounted for. Even thermistors can be embedded in a SiPM to be more representative of SiPM temperature, it may not account for the gain error caused by the other factors mentioned above. Thus, there is a need to improve gain compensation in detectors for imaging systems by measuring the gain directly.